It is hypothesized that the lethal response of cells to fractioned hyperthermia is strongly influenced by variations in the cellular microenvironment and by cell line specific characteristics which differentiate tumor and normal tissue in vivo. The overall objective of this proposal is to generate fundamental and clinically relevant information relating the response of mammalian cells to fractionated hyperthermia with (1) temperature and duration of heat treatment, (2) cell-line specific thermal sensitivity, (3) variation in selected extracellular fluid metabolites such as oxygen and pH, and (4) cell generation time. Under these multiple variable conditions, cellular resistance to hyperthermia, induced by prior heat treatment (thermal tolerance), will be evaluated with respect to its magnitude, rate of development and rate of decay. The response of cell to fractioned X-irradiation and hyperthermia will be compared to determine if their effects are similar or complementary. The cell-line specificity of the magnitude and kinetics of thermal resistance will be ascertained by using four cell lines of widely differing thermal sensitivities and cell-cycle times. Oxygen concentration and pH will be varied during treatments only, between temperatures, and in various combinations. The temperature dependency of any sensitizing effects will be determined. The effect of post-heat cell division on the rate of decay of thermotolerance is to be evaluated using cell lines of different generation time and by maintaining cells in nutritionally and density-induced plateau phase for various periods of time. Variations in thermal sensitivity due to cell redistribution and nutritional depletion between fractionated heat treatments will be determined and controlled. The principles derived from these studies will be of practical value in planning clinical hyperthermia trials for the treatment of cancer.